1. Field of the Invention
This invention relates to a bistable reflective display and methods of forming the same. In particular, the invention relates to a bistable reflective display using chiral liquid crystal and silica agglomerates and methods of forming the same.
2. Description of Related Art
Several types of reflective liquid crystal displays have recently been developed. Many of these reflective liquid crystal displays utilize liquid crystal-polymer dispersion technologies. Such displays are superior to conventional polarizer based displays for reflective mode operation.
An example of one type of such a reflective liquid crystal display is the polymer-dispersed liquid crystal (PDLC) display, which operates on the principle of electrically controlled light scattering. With this technology, liquid crystal droplets are embedded in a polymer matrix. In the off-state, the alignment of the liquid crystal droplets (symmetry axis) is random, resulting in an opaque, scattering film because of the mismatch between effective refractive index of the liquid crystal with that of the polymer. Upon application of an electric field, the liquid crystal within the liquid crystal droplets aligns parallel to the electric field and the composite material becomes transparent. However, contrast ratios in the direct-view reflective mode are in the 5-10:1 range which are strongly cell thickness dependent. Further, the reflectivity of the polymer dispersed liquid crystal reflective display is only about 12 to 15%.
Another type of reflective liquid crystal display is the polymer dispersed cholesteric liquid crystal display (PDCLC), which operates on the principle of Bragg reflection. Such cholesteric liquid crystal displays have a contrast ratio approaching approximately 10:1 with a photopic reflectivity of 10-13% under ambient lighting conditions and approximately 40% at the Bragg wavelength.
Another type of reflective liquid crystal display is a polymer stabilized cholesteric texture (PSCT) reflective display. The polymer stabilized cholesteric texture reflective display uses a small amount of polymer additive in the cholesteric liquid crystal medium which assembles into a stabilizing network. Contrast ratios have been reported between 20-30:1 with 10 to 15% photopic reflection under ambient lighting conditions, and nearly 40% at the Bragg wavelength. Similar displays have been demonstrated without the polymer with comparable performance.
A recent type of reflective liquid crystal display is a stabilized liquid crystal display formed with nematic phase liquid crystal stabilized by silica agglomerates dispersed within the liquid crystal. Use of the silica agglomerates allows switching the display between a transparent state and a light scattering state both of which are stable without maintaining the voltage (or electric field) used to switch from the transparent to the light scattering state. However, transition from the transparent state to a reflective state requires thermal recycling, with laser addressing or perhaps through the use of ultrasound.
Another recent type of reflective liquid crystal display is the holographic polymer dispersed liquid crystal display. Such a display is reported in "Holographically formed liquid crystal/polymer device for reflective color displays", by Tanaka et al., as reported in the Journal of the Society for Information Display, Volume 2, No. 1, 1994, pages 37-40. Further developments by Tanaka et al. reported on optimization of such a holographic liquid crystal display in "Optimization of Holographic PDLC for Reflective Color Display Applications" in the SID '95 Digest, pages 267-270. This holographically formed polymer dispersed liquid crystal is formed using optical interference techniques to form planes of liquid crystal droplets at predesignated positions within the sample setting up a modulation in the liquid crystal droplet densities. The resulting optical interference reflects the Bragg wavelength in the off-state when the droplets are misaligned. Upon application of an applied voltage, the periodic refractive index modulation vanishes if the refractive index of the liquid crystal is approximately matched with the refractive index of the polymer, and all incident light is transmitted. The spectral reflectance of the display is determined during the fabrication process and can be chosen to reflect any visible wavelength. The above-described holographic liquid crystal/polymer reflective color display is formed with an isotropic polymer which results in liquid crystal droplets being formed during the phase separation. Because the polymer is isotropic, the molecules of the polymer are randomly aligned and the display device has visible opaqueness or haze when viewed from an angle due to the mismatch between the effective refractive index of the liquid crystal and that of the polymer becomes enhanced at wide angles. Additionally, this display device requires a relatively large drive voltage due to the liquid crystal spherical droplets. In particular, the voltage necessary to drive the display device is proportional to the surface-to-volume ratio of the liquid crystal droplets. Such spherical droplets have a surface-to-volume ratio of 3/R where R is the radius of the droplet.
U.S. patent application attorney docket No. JAO 34133, entitled "HOLOGRAPHICALLY FORMED REFLECTIVE DISPLAY, LIQUID CRYSTAL DISPLAY AND PROJECTION SYSTEMS AND METHODS OF FORMING THE SAME", the subject matter of which is incorporated herein, discloses holographically formed reflective displays and projection systems. U.S. patent application Ser. No. 08/792,269, entitled "BROADBAND REFLECTIVE DISPLAY AND METHODS OF FORMING THE SAME", the subject matter of which is incorporated herein, discloses broadening the reflective wavelengths by including nine or more groups of reflective layers each being reflective of different wavelengths of light.
Accordingly, there is a need to provide a bistable reflective display that has an improved photopic white reflectance, can operate at reduced drive voltages, has a high contrast and has a haze free appearance when viewed from different viewing angles.